SBS633 EvolutionaryGenetics Snail Day Trip Report Introduction
Polymorphism is a phenomenon defined as different alleles occurring at the same location and the same period of time. The evolutionary processes that play a role in polymorphism are selection, genetic drift (changes in allele frequency due to chance), gene flow (movement of alleles from a population to another) and mutation. In this experiment, we studied the morph frequencies in populations of the extremely polymorphic brown-lipped snails Cepaea nemoralis and we established a correlation between altitude and frequency of phenotypes.
Cepaea nemoralis is a species of land snails mostly found in Europe and is mainly used for genetics and evolutionary studies. These snails are considered good model organisms as they ‘’carry their genes on their shells and are highly polymorphic’’ (Jones et al, 1977). Traits include shell colour (yellow, pink or brown) and shell banding pattern (unbanded, middle banded and many banded). These snails are easier to study than humans mainly due to their small size and limitedmobility. As a result, genetic patterns will be limited to a small area thus enabling us to collect them in one locality and then studying their populations could be much easier unlike humans where gene flow is rather high.
The aim of this experiment, which was carried out in Pulpit hill near Monk’s Riseborough, is to examine the power of evolutionary processes such as genetic drift, selection and gene flow on the polymorphic distribution of alleles in Cepaea nemoralis and to see which of these processes has the strongest effect on thefrequency of alleles. Our sampling design enabled us to compare the phenotype of snails in different locations and consider what evolutionary process causes the observed samples.
The experimental design was carried out by observing the shell colour and band pattern of Cepeae nemoralis snails from different locations. Our group collected a minimum of thirty samples of snails from two different altitudes, uphill and downhill. Snails were collected from 4 different sites in those 2 altitudes (8 sites in total). Then, different variants of snails (shell colour and banding pattern) were recorded and a table was formed for comparison. The differences could be due to genetic drift, selection or gene flow. Our null hypothesis is that there would be no differences between phenotypic frequencies of the snails found at different altitudes.
If the polymorphism of snails is caused by natural selection, then phenotype frequency will vary with different types of habitats. Genetic drift may well also explain the differences in allele frequency between any pair of areas. For example, genetic drift could increase the frequency of a phenotype in one area, and of another phenotype in another area. However, it will not cause a constant association between a certain locality and the frequency of alleles.
The findings from this experiment will be analysed using a statistical chi squared test to see if differences are small enough to be recognized by sampling error and if the results indicate whether there is a significant difference between the phenotypes found between different locations. If differences were due to genetic drift or other random process, we would notice significant differences in phenotype frequencies.
Results The tables below provide a summery of the collected results by chi-squared tests to prove statistical significance between the different species and sites. Results are further discussed related to the proposed hypothesis deliberating any significance in the data between the upper and lower sites in the observed phenotypes.
Figure 1- A total number of coloured phenotype observed uphill and the results of the chi- square statistical test.
uphill
yellow
Pink &brown
total
Site 1
5 Ex= 4.88 X2= 0.0029
8 Ex=8.12 X2= 0.0018
13
Site 2
7 Ex= 12.75 X2= 2.5931
27 Ex=21.25 X2=1.5559
34
Site 3
25 Ex= 13.88 X2= 8.9088
12 Ex=23.12 X2 = 5.3484
37
Site 4
2 Ex= 7.5 X2= 4.0333
18 Ex=12.5 X2= 2.42
20
total
39
65
104
d.o.f = (4-1)*(2-1) = 3*1 = 3 Total x2 = 24.86 > 11.34 Larger that the critical x2 0.01 value. We reject the null hypothesis.
Figure 2:A total number of banded phenotype observed uphill and the results of the chi- square statistical test.
uphill
unbanded
Many banded
Total
Site 1
7 Ex= 2.79 X2 = 6.3527
6 Ex=10.21 X2= 1.7350
13
Site 2
7 Ex= 7.29 X2= 0.0115
27 Ex= 26.71 X2= 0.0031
34
Site 3
1 Ex= 6.86 X2= 5.0058
31 Ex= 25.14 X2= 1.3659
32
Site 4
6 Ex= 4.07 X2=0.9152
13 Ex= 14.93 X2= 0.2495
19
total
21
77
98
d.o.f = (4-1)*(2-1) = 3*1 = 3 Total x2 =15.64 > 11.34 Larger that the critical x2 0.01 value. We reject the null hypothesis
Figure 3- A total number of coloured phenotype observed downhill and the results of the chi- square statistical test.
downhill
yellow
Pink &brown
Total
Site 1
34 Ex= 36.76 X2= 0.2072
27 Ex=24.24 X2= 0.3143
61
Site 2
14 Ex= 18.08 X2= 0.9207
16 Ex=11.92 X2=1.3965
30
Site 3
24 Ex= 21.69 X2= 0.2460
12 Ex=14.30 X2 = 0.3699
36
Site 4
22 Ex= 17.47 X2= 1.1746
7 Ex=11.53 X2= 1.7798
29
total
94
62
156
d.o.f = (4-1)*(2-1) = 3*1 = 3 Total x2 = 6.409 < 11.34 smaller that the critical x2 0.01 value .We accept the null hypothesis
Figure 4- A total number of banded phenotype observed downhill and the results of the chi- square statistical test.
downhil
unbanded
Many banded
Total
Site 1
4 Ex= 8.21 X2 = 2.1588
57 Ex=52.79 X2= 0.3357
61
Site 2
2 Ex= 4.04 X2= 1.0300
28 Ex= 25.96 X2= 0.1603
30
Site 3
11 Ex= 4.85 X2= 7.7985
25 Ex= 31.15 X2= 1.2142
36
Site 4
4 Ex= 3.90 X2=0.0026
25 Ex= 25.10 X2= 0.0004
29
total
21
135
156
d.o.f = (4-1)*(2-1) = 3*1 = 3 Total x2 = 12.70 > 11.34 Larger that the critical x2 0.01 value. We therefore reject the null hypothesis.
Discussion
Results from 4 different sites of high and low altitudes were noted according the snails shell colour (yellow, pink and brown) and banding pattern (unbanded and many banded), where the majority of the snails that was collected were dead. Chi squared tests were conducted to demonstrate if the differences in allele frequency were due to genetic drift or selection, and estimates whether the difference in phenotype frequencies is statistically substantial. Results indicated a significant difference between the colour and band of C. Nemoralis, located at high or low altitudes as we have failed to accept the null hypothesis.
Figure 1 and 2 rejects the null hypothesis where totals of x2 = 24.86 in Figure 1 and x2 =15.64 in Figure 2 were found to be larger than the critical value, demonstrating a greater abundance of darker coloured phenotypes (pink and brown) and many banded snails in the uphill sample.
Figure 3 and 4 analyse the collected downhill results of the observed phenotypes. Figure 3 found no significant difference in the coloured phenotypes, showing a total of x2 = 6.409 being smaller than the critical value, thus accepting the null hypothesis. The null hypothesis was rejected in figure 4, where analysis of the banded phenotype found a total of x2 = 12.70 to be greater than the critical value.
Results showed that within the uphill samples, there was found to be a greater expected value for many banded snails, and similarly the many banded snails were observed in the downhill samples. In this essence, taking account the coloured phenotype, yellow unbanded snails were found to have no statistical difference between the two regions, however yellow snail observations in general were found more prominently downhill. From these results, we can assume that at lower altitudes, there is less exposure to the sun and so there are fewer unbanded snails, this is because the more bands the snail has, the darker the shell will be, allowing it to acquire energy from the sun at a higher rate.
However, limitations in our experiment may be due to sampling errors that affect our results, and could be reduced by increase the sampling size and replication of the study, i.e repeating the experiment many times. The experiment could have benefitted if we went on a rainy day, as opposed to the fairly dry day when the experiment was conducted, as the ideal environment for snails is rainy and wet, which can explains why most snails we found were dead. Furthermore, the studied area was open to public; hence the collected samples may not be an accurate representation of the snail population. Finally, our collected samples may have been more accurate with the use of a quadrant.
In conclusion, the results indicate the significant differences can be attributed to factors involved in natural selection as indicated above, however these observed differences can also be found to be due to genetic drift. Research supporting this by Jones et al. (1977) suggested that these differences in morph frequencies can be due to genetic drift.
References
Jones, J. S., Leith, B. H. and Rawlings, P. (1977). Polymorphism in Cepaea: a problem with too many solutions? Ann Rev Ecol Syst, 8: 109–143.
Snail Day Trip Report
Introduction
Polymorphism is a phenomenon defined as different alleles occurring at the same location and the same period of time. The evolutionary processes that play a role in polymorphism are selection, genetic drift (changes in allele frequency due to chance), gene flow (movement of alleles from a population to another) and mutation. In this experiment, we studied the morph frequencies in populations of the extremely polymorphic brown-lipped snails Cepaea nemoralis and we established a correlation between altitude and frequency of phenotypes.
Cepaea nemoralis is a species of land snails mostly found in Europe and is mainly used for genetics and evolutionary studies. These snails are considered good model organisms as they ‘’carry their genes on their shells and are highly polymorphic’’ (Jones et al, 1977). Traits include shell colour (yellow, pink or brown) and shell banding pattern (unbanded, middle banded and many banded). These snails are easier to study than humans mainly due to their small size and limitedmobility. As a result, genetic patterns will be limited to a small area thus enabling us to collect them in one locality and then studying their populations could be much easier unlike humans where gene flow is rather high.
The aim of this experiment, which was carried out in Pulpit hill near Monk’s Riseborough, is to examine the power of evolutionary processes such as genetic drift, selection and gene flow on the polymorphic distribution of alleles in Cepaea nemoralis and to see which of these processes has the strongest effect on thefrequency of alleles. Our sampling design enabled us to compare the phenotype of snails in different locations and consider what evolutionary process causes the observed samples.
The experimental design was carried out by observing the shell colour and band pattern of Cepeae nemoralis snails from different locations. Our group collected a minimum of thirty samples of snails from two different altitudes, uphill and downhill. Snails were collected from 4 different sites in those 2 altitudes (8 sites in total). Then, different variants of snails (shell colour and banding pattern) were recorded and a table was formed for comparison. The differences could be due to genetic drift, selection or gene flow. Our null hypothesis is that there would be no differences between phenotypic frequencies of the snails found at different altitudes.
If the polymorphism of snails is caused by natural selection, then phenotype frequency will vary with different types of habitats. Genetic drift may well also explain the differences in allele frequency between any pair of areas. For example, genetic drift could increase the frequency of a phenotype in one area, and of another phenotype in another area. However, it will not cause a constant association between a certain locality and the frequency of alleles.
The findings from this experiment will be analysed using a statistical chi squared test to see if differences are small enough to be recognized by sampling error and if the results indicate whether there is a significant difference between the phenotypes found between different locations. If differences were due to genetic drift or other random process, we would notice significant differences in phenotype frequencies.
Results
The tables below provide a summery of the collected results by chi-squared tests to prove statistical significance between the different species and sites. Results are further discussed related to the proposed hypothesis deliberating any significance in the data between the upper and lower sites in the observed phenotypes.
Figure 1- A total number of coloured phenotype observed uphill and the results of the chi- square statistical test.
Ex= 4.88
X2= 0.0029
Ex=8.12
X2= 0.0018
Ex= 12.75
X2= 2.5931
Ex=21.25
X2=1.5559
Ex= 13.88
X2= 8.9088
Ex=23.12
X2 = 5.3484
Ex= 7.5
X2= 4.0333
Ex=12.5
X2= 2.42
Total x2 = 24.86 > 11.34
Larger that the critical x2 0.01 value. We reject the null hypothesis.
Figure 2:A total number of banded phenotype observed uphill and the results of the chi- square statistical test.
Ex= 2.79
X2 = 6.3527
Ex=10.21
X2= 1.7350
Ex= 7.29
X2= 0.0115
Ex= 26.71
X2= 0.0031
Ex= 6.86
X2= 5.0058
Ex= 25.14
X2= 1.3659
Ex= 4.07
X2=0.9152
Ex= 14.93
X2= 0.2495
Total x2 =15.64 > 11.34
Larger that the critical x2 0.01 value. We reject the null hypothesis
Figure 3- A total number of coloured phenotype observed downhill and the results of the chi- square statistical test.
Ex= 36.76
X2= 0.2072
Ex=24.24
X2= 0.3143
Ex= 18.08
X2= 0.9207
Ex=11.92
X2=1.3965
Ex= 21.69
X2= 0.2460
Ex=14.30
X2 = 0.3699
Ex= 17.47
X2= 1.1746
Ex=11.53
X2= 1.7798
Total x2 = 6.409 < 11.34
smaller that the critical x2 0.01 value .We accept the null hypothesis
Figure 4- A total number of banded phenotype observed downhill and the results of the chi- square statistical test.
Ex= 8.21
X2 = 2.1588
Ex=52.79
X2= 0.3357
Ex= 4.04
X2= 1.0300
Ex= 25.96
X2= 0.1603
Ex= 4.85
X2= 7.7985
Ex= 31.15
X2= 1.2142
Ex= 3.90
X2=0.0026
Ex= 25.10
X2= 0.0004
Total x2 = 12.70 > 11.34
Larger that the critical x2 0.01 value. We therefore reject the null hypothesis.
Discussion
Results from 4 different sites of high and low altitudes were noted according the snails shell colour (yellow, pink and brown) and banding pattern (unbanded and many banded), where the majority of the snails that was collected were dead. Chi squared tests were conducted to demonstrate if the differences in allele frequency were due to genetic drift or selection, and estimates whether the difference in phenotype frequencies is statistically substantial. Results indicated a significant difference between the colour and band of C. Nemoralis, located at high or low altitudes as we have failed to accept the null hypothesis.
Figure 1 and 2 rejects the null hypothesis where totals of x2 = 24.86 in Figure 1 and x2 =15.64 in Figure 2 were found to be larger than the critical value, demonstrating a greater abundance of darker coloured phenotypes (pink and brown) and many banded snails in the uphill sample.
Figure 3 and 4 analyse the collected downhill results of the observed phenotypes. Figure 3 found no significant difference in the coloured phenotypes, showing a total of x2 = 6.409 being smaller than the critical value, thus accepting the null hypothesis. The null hypothesis was rejected in figure 4, where analysis of the banded phenotype found a total of x2 = 12.70 to be greater than the critical value.
Results showed that within the uphill samples, there was found to be a greater expected value for many banded snails, and similarly the many banded snails were observed in the downhill samples. In this essence, taking account the coloured phenotype, yellow unbanded snails were found to have no statistical difference between the two regions, however yellow snail observations in general were found more prominently downhill. From these results, we can assume that at lower altitudes, there is less exposure to the sun and so there are fewer unbanded snails, this is because the more bands the snail has, the darker the shell will be, allowing it to acquire energy from the sun at a higher rate.
However, limitations in our experiment may be due to sampling errors that affect our results, and could be reduced by increase the sampling size and replication of the study, i.e repeating the experiment many times. The experiment could have benefitted if we went on a rainy day, as opposed to the fairly dry day when the experiment was conducted, as the ideal environment for snails is rainy and wet, which can explains why most snails we found were dead. Furthermore, the studied area was open to public; hence the collected samples may not be an accurate representation of the snail population. Finally, our collected samples may have been more accurate with the use of a quadrant.
In conclusion, the results indicate the significant differences can be attributed to factors involved in natural selection as indicated above, however these observed differences can also be found to be due to genetic drift. Research supporting this by Jones et al. (1977) suggested that these differences in morph frequencies can be due to genetic drift.
References
Jones, J. S., Leith, B. H. and Rawlings, P. (1977). Polymorphism in Cepaea: a problem with too many solutions? Ann Rev Ecol Syst, 8: 109–143.
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